A. Field of the Invention
This invention relates to a process for embossing a man-made board, and more particularly to a process for embossing or debossing a man-made cellulosic board at a high temperature and pressure after consolidation of the board. More specifically, the process of the present invention relates to embossing a man-made cellulosic board in a post-press embossing step.
Man-made consolidated boards include hardboard, chip board, particle board, panel board, acoustical board, insulation board and the like. The uses of such boards depend upon the physical characteristics such as the density of the board and the exterior embossing or decorative effect applied thereto. The qualities and physical characteristics of a board depend upon the conditions under which the board is manufactured, and the raw materials used to form the board.
There are two principal processes for the manufacture of man-made boards--the wet process and the dry process. In the wet process, the raw material is uniformly blended in a head box with copious quantities of water to form a slurry. The slurry is deposited onto a water-pervious support member, generally a Fourdrinier wire, where much of the water is removed leaving a wet mat of cellulosic material. The wet mat is transferred from the pervious support member and consolidated under heat and pressure to form the board. Typically, pressures of from 400 to 500 psi and temperatures up to about 400.degree. F. are encountered in consolidation of a man-made board manufactured by the wet process.
The dry process is similar to the wet process except that the cellulosic fibers are first coated with a thermosetting resin binder, such as a phenol-formaldehyde resin, and are then randomly distributed into a mat by air blowing the resin-coated fibers onto a support member. In the dry process, the mat is pressed at temperatures up to about 450.degree. F. and pressures less than about 1000 psi to cure the thermosetting resin and to compress the mat into an integral consolidated structure.
The process of embossing a wet mat in a platen press with a heated embossing plate is well known, especially in the manufacture of hardboard. An embossing plate is made with a surface contour of a desired design, such as wood graining. The heated embossing plate is pressed against the surface of the wet mat under sufficient pressure to impress the plate design into the surface of the panel and thereby consolidate the wet mat into a decorative man-made board.
Boards made in accordance with the dry process are also typically decorated or surface-designed by hot-pressing in a platen press with a heated embossing plate. In embossing mats made by either the wet or dry process, however, the man-made mat is embossed concurrently with being compressed into a consolidated, integral product so that the bonds between fibers are initially made in accordance with the final surface design of the product.
Much of the bonding necessary for cohesion and strength in a man-made board occurs during the consolidation of the board. Prior to drying a board made by the wet process, the board is weak, but the bonding forces created during the final consolidation are generally quite powerful. Generally the same bonding effect occurs in the final consolidation of the board made by the dry process. Prior to hot-pressing, the loosely disassociated cellulosic fibrous product is quite weak, but after hot-pressing the mat into its final configuration, it is very powerfully held together by the bonding which occurs during hot-pressing. Hot-pressing during consolidation causes a welding or coalescing of the cellulose fibers at the surface of the product so that the surface portion consists of wood re-made in modified form. It is very difficult to re-shape the surface of the consolidated product without again destroying these fiber-to-fiber surface welds.
As set forth above, the process of embossing a manmade board prior to consolidation to establish a surface design in a product is well known. However, because of the strong inter-fiber bonding which occurs during consolidation, no effective process has been developed by which a previously consolidated product can be post-press embossed to create a surface design or to re-design the surface of a consolidated product. To be effective, such a process cannot destroy the inter-fiber bonding established at the surface of the board during consolidation--yet the process must permanently re-shape the surface of the product.
One of the most difficult problems found by applicant in re-shaping the surface of a consolidated product is in maintaining "design fidelity" in the board. "Design fidelity" or simply "fidelity" as used herein is a measure of the accuracy of reproduction of the design of the embossing plate onto the surface of the consolidated board. Because of the strong interfiber bonding referred to above existing in a consolidate board, the re-shaped surface has a tendency to "spring-back" to its original surface design. "Spring-back" as used herein is the tendency of a re-shaped board to return to its original surface shape or contour.
No existing process is able to maintain a high degree of fidelity over extended periods of time by embossing a consolidated product because of the strong inter-fiber bonding occuring during initial consolidation.
The present invention relates to a process of embossing a consolidated man-made cellulosic board in such a manner that the strong inter-fiber bonding established during consolidation and densification is maintained, but the cellulosic material is apparently caused to "flow" to new locations to impart a different contour to the surface of the consolidated product. A process whereby a consolidated product can be embossed under heat and pressure, after already having undergone a first hot-press consolidation step, so that the inter-fiber bonds established during the first hot-press step are maintained, but re-oriented with the fibrous material in the new shape, is quite surprising in the art of man-made cellulosic board manufacture.
In accordance with the process set forth herein, a consolidated product is post-press embossed to establish new surface contours. The inter-fiber bonds established in initial consolidation are realigned to set the surface of the product permanently into a new design. The re-shaping is very effective so that very little spring-back occurs.
B. Prior Art
Many patents disclose hot-pressing as a step for consolidating a composite board into its final shape. Some of the hot-pressing consolidation procedures are disclosed in the following U.S. Pat. Nos.: 3,113,065 Newton: 2,388,487 and 3,002,878 Linzell: 2,888,376 Stewart: 3,428,727 Higgins; 3,758,357 Akerson; 3,788,929 Huttunen; 2,298,017 Loughborough; 2,803,188 Duvall; 3,681,115 Maher; 2,495,043 Willey et al; 3,699,203 Oshima et al; and 3,718,536 Downs. Some of the above patents also disclose the chemical treatment of the fibrous product prior to hot-pressing to enhance one or more properties of the final product. The following U.S. Pat. Nos. relate to the chemical treatment of a fibrous product: 2,811,470 Kenaga; 2,724,642 Brown; 2,692,183 Ericks; 2,395,311 Woodhouse; and 1,948,314 Pratt. Of the above patents, the U.S. Pat. Nos. 3,758,357 Akerson et al; 3,788,929 Huttenen and 2,298,017 Loughborough disclose hot-pressing a wood product for the purpose of bending the product. However, none of these three patents discloses post-press embossing with a contoured embossing plate and therefore do not encounter the fidelity problems encountered in re-shaping a previously consolidated product. The above-listed U.S. Pat. Nos. Downs 3,718,536 and Oshima 3,699,203 relate to a second hot-press step, but each patent discloses bonding the product into its final shape by providing a thermosettable plastic which sets on hot-pressing. The U.S. Pat. No. 2,803,188 Duvall relates to a hot-press embossing step after consolidation of an insulation board, but provides a hardenable coating composition so that spring-back over extended periods of time is not a significant factor.